A typical thermal printer creates marks on a print medium by selectively heating elements within a thermal printhead to cause the transfer of ink from a thermal printer ribbon to the print medium.
As seen in FIG. 1, a conventional thermal printhead 2 used for bar-code printing typically comprises an array of small thermal print elements 2a, each of which produces heat in response to an electrical input signal. The smallest element that can be printed, termed a pixel, is dependent on the size of the thermal print elements 2a. Each thermal print element 2a is typically a resistive strip of thermal material through which an electrical current is passed. In some thermal printer applications, such as a bar code thermal printer, the thermal print elements 2a are arranged in a linear array four to six inches wide with 800-1200 thermal print elements in a 1.times.800 or 1.times.1200 array. In such applications, the thermal printhead 2 is stationary and a print medium 6 moves past the thermal printhead.
In a typical bar code thermal printer, the print medium 6 moves in a transverse direction past the thermal printhead 2 containing the linear array of thermal print elements 2a. The print medium 6 is in thermal contact with the thermal print elements 2a as it is moved past the thermal printhead 2 in a stepwise fashion. During each step, desired thermal print elements 2a are selectively heated and portions of the print medium 6 in thermal contact with the heated thermal print elements are darkened from ink transferred from a thermal printer ribbon 4 to the print medium.
The print medium 6 is in thermal contact with the thermal printhead 2 for a predetermined period of time designated as a scan line time, or SLT. A given SLT may be further broken down into multiple time segments, allowing portions of the SLT to be processed separately. In a typical thermal printhead 2, a print command signal is input to each thermal print element 2a selected to print during a particular SLT. The print command signal is designed to raise the temperature of the thermal print element 2a to a prescribed temperature and to maintain the temperature level for a prescribed time. In its most simple form, the print element is energized at a constant level during the entire SLT if printing is desired, and is not energized at all if no printing is desired.
It is well known in the art that the ambient temperature of the thermal printhead 2 can affect the quality of the printing. For example, if the thermal printhead 2 has a relatively high ambient temperature, the image transferred to the print medium 6 appears to be enlarged relative to the same image printed with the thermal printhead 2 at a relatively low ambient temperature. This effect is due to the residual heat of the thermal print elements 2a causing the transfer of an excessive amount of ink from the thermal printer ribbon 4 to the print medium 6.
In more sophisticated thermal printers, the print command signal is a logical AND combination of data signal and a strobe signal. The logical AND of the data signal and the strobe signal controls whether or not thermal print element 2a will be heated at any particular time. This signal will be referred to herein as an energization signal. It is known in the art to use the strobe signal to compensate for variations in the ambient temperature of the thermal printhead 2 over a relatively long period of time. For example, when the thermal printer initially begins operation, the ambient temperature of the thermal printhead 2 is relatively low. Thus, the strobe signal may be longer in duration to allow the proper transfer of heat to the thermal print elements 2a to transfer a desired amount of ink from the thermal printer ribbon 4 to the print medium 6. As the ambient temperature of the thermal printhead 2 increases during the course of a print job or during the day, the strobe signal may be altered so as to transfer less energy to the thermal print elements 2a in order to transfer the same desired amount of ink from the thermal printer ribbon to the print medium 6. If no such compensation were incorporated, pixels printed during the warm-up period would be lighter than desired due to insufficient heat being transferred to the print element 2a during the SLT. After the printhead is warmed up, the pixels would be darker than desired due to the residual heat in each print element 2a.
Even with the long-term compensation for the ambient temperature of the thermal printhead 2, thermal printers of the prior art cannot compensate for changes in the quality of the thermal printer ribbon 4 itself. In a multipass thermal printer ribbon 4, the print quality is affected by the number of times in which the thermal printer ribbon 4 is used, as well as the amount of ink transferred from the thermal printer ribbon to the print medium 6 during previous passes. For example, FIG. 1 illustrates the transfer of a portion 8a of ink from the thermal printer ribbon 4 to the print medium 6. The thermal printer ribbon 4 has a corresponding indentation 8b where ink from the thermal printer ribbon was transferred to the print medium 6. Thus, the print quality is affected by the amount of ink removed from the thermal printer ribbon 4 in previous passes.
Previous efforts to improve multipass thermal ribbon technology have focused on changing the chemical and physical composition of the ribbon itself. Therefore, it can be appreciated that there is a significant need for a thermal printer that can compensate for variations in the multipass thermal printer ribbon in order to maximize the print quality. The present invention provides this and other advantages as will be seen by way of the accompanying drawings and detailed description.